2 Why We Need Model-Based Testing

104 Exploring and Analyzing Finite Model Programs
ModelProgramSimple m = ModelName.Factory.Create(); // factory method
Sequence frontier = new Sequence(m.InitialState);
Sequence explored = new Sequence(); // empty
Set transitions = new Set(); // empty
while (!frontier.IsEmpty)
{
State current = frontier.Head; // choose first element of frontier
frontier = frontier.Tail; // all but first element of frontier
explored = explored.AddLast(current); // append current to explored
foreach (Action a in m.GetActions(current))
{
State next = m.GetNextState(current, a);
if (!frontier.Contains(next) && !explored.Contains(next))
{
frontier = frontier.AddLast(next); // append for breadth-first
}
transitions = transitions.Add(Transition(current, a, next));
}
}
Figure 6.7. An exhaustive exploration algorithm.
The exploration algorithm also uses these types, which are defined in the modeling
library (Chapter 10):
Set The unordered collection of elements of type T, with constructors
Set(), Set(x,y,z), and so on, and method Add, where s.Add(x) returns
a new set that contains all the elements of set s and the element x.
Sequence The ordered collection of elements of type T, with properties Head
(the first element), Tail (all but the first element), and methods AddFirst (which
pushes an element on the head) , and AddLast (which appends an element to
end).
Figure 6.7 shows an exhaustive exploration algorithm. (This simplified version
is not the actual code used by the mpv tool.) The frontier is the collection of states
that have been reached but whose enabled transitions have not yet been executed.
(In Figure 6.5, states 1 and 2 are on the frontier; in Figure 6.6, states 1, 3, and 4
are on the frontier.) When exploration begins, only the initial state is on the frontier.
Exploration proceeds by executing transitions that are enabled on the frontier.
When executing, a transition reaches a state that has not already been reached, that
state is added to the frontier. When all of the transitions that are enabled in a state
have been executed, that state is removed from the frontier and added to the collection
of explored states. (In Figure 6.5, state 0 has been explored; in Figure 6.6,
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